The term climate change is often used interchangeably with global warming. But while overall global temperatures are warming, the effects of climate change are not limited to hotter temperatures. (In fact, some locations are forecasted to get cooler.) A historically destructive wildfire season plagued California, while heavy storms set July rainfall records on the east coast of the U.S. That's not to mention 2018's hurricane season. Understanding the underlying statistics can help explain why all the repercussions of climate change can look so different.

So if Earth is getting warmer, where do all of the recent natural disasters and extreme weather events fit into the picture?

This past July was a key example of this kind of variability: record-setting highs across the globe were coupled with devastating extreme weather events like monsoons in India and flooding in South Carolina. Imagine climate as a bell curve, where the height on the curve indicates how often something occurs. Natural things often have this shape, called a normal distribution — examples include adult height, the dimensions of almonds, and proteins found in human blood are all roughly normally distributed. This means that more common values tend to be closer to the average (also called the mean), while much larger and much smaller values are less numerous. For example, the mean height for men is 5’9”, so it’s more likely for men to be between 5’6” and 6’0” tall than it is to meet someone who is 7’ tall.

Normal.

Mullookkaaran via Wikimedia

The distance values tend to be from the average is called the variance. In distributions with a “flatter” shape, values far away from the average are more likely than in a traditional bell curve. Another way of saying that is that unlikely events are more common in flat distributions, or those with a large variance. For example, if we lived in a hypothetical world with a lot of 4’ and 7’ tall men, the variance of that population’s height would be much larger than the one we actually observe.

There would be more hot weather, including more record hot weather, and far less cold weather than its current climate.

Because these curves are easy to understand and translate, climate scientists often use a normal distribution to visualize certain parts of the climate, like temperature, precipitation, and the carbon cycle. These models indicate that most days will have weather closer to average (think "normal" spring and fall weather), while extreme hot and extreme cold weather events happen less often. So, if the Earth is getting warmer, where do all of the recent natural disasters and extreme weather events fit into the picture?

Consider, for instance, the distribution of climate in Nashville, Tennessee — summers are hot, winters are cold, and the average yearly temperature is 59°F. Now, imagine if Nashville suddenly became as warm as Houston, Texas, where by comparison, summers and winters are hotter and the average yearly temperature is 69°F. While temperatures are increased, such a change would not necessarily impact its variance — meaning summers would still be comparatively hotter than winters. Figure 1 (below) visualizes what would happen in this new Nashville climate: There would be more hot weather, including more record hot weather, and far less cold weather than its current climate.

Figure 1:Schematic outlining the effect of an increased mean on the climate and subsequent weather patterns, from the IPCC (2001)

Coleman Harris

Now, what happens if the average temperature stays the same, but the variance increases? Climate change, after all, doesn't just increase temperatures, it increases the possibility of record-setting heat, cold, rain, and associated natural disasters. (Our current curve, in other words, gets much flatter.) Nashville, for example, is already one of the more variable cities in the United States, with average annual high temperatures of 70°F, and average annual low temperatures of 49°F, a difference of 21°F. This explains why Nashville has freezing winters that drop into the 20s and scorching summers that reach into the 90s. Compare this with a less variable place like that of San Diego, California, which is always warm, or Juneau, Alaska, which is always comparatively cold. Increasing variance, as shown in Figure 2 below, creates record hot and record cold weather into the climate — better pack both a jacket and swimsuit.

Figure 2:Schematic outlining the effect of an increased variance on the climate and subsequent weather patterns, from the IPCC (2001)

Coleman Harris

So, what happens when we combine the two effects? There are not many real-world examples of this happening yet, but Figure 3 (below) shows what we have to look forward to. Average temperatures will get hotter, as well as having more record-breaking heat spikes, as well as far less cold weather. Looking at Nashville again, consider that hot weather would now occupy a much larger portion of Nashville’s bell curve—but temperature is only one factor in observable climate. There are dozens of systems impacted by climate change, such as precipitation, the carbon cycle, oceans, glaciers, and the atmosphere. Each of these has its own well-balanced distribution that is currently threatened by changes to the Earth’s climate. By changing the variance within each of these factors, the changing climate poses a serious threat through powerful natural disasters that have potential to impact millions.

Figure 3: Schematic outlining the effect of an increased mean & increased variance on the climate and subsequent weather patterns, from the IPCC (2001)

Coleman Harris

Increases to the mean and the variance have already impacted the world today, visible in our recent wildfires, flooding, and heatwaves. While each major disaster cannot be directly attributed to climate change, the increasing intensity and frequency of natural disasters is attributable to changes in the Earth’s climate. If measures are not enacted to curb climate change soon, Earth will continue on its path out of balance — resulting in potentially permanent damage to the planet and a climate distribution unfit for human life.

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

I follow all of these conceptual figures and it makes perfect sense logically, but are there peer-reviewed studies that actually have found that temperature or other environmental factors are becoming more variable? To me, this piece feels like its talking about a hypothetical scenario (albeit a likely one) instead of something that we know is actually happening.

If there are studies or news articles that provide real-world examples about how climate change is making things more variable, it would be good to talk about those to root some of these conceptual figures in reality. This sentence: “Increases to the mean and the variance have already impacted the world today, visible in our recent wildfires, flooding, and heatwaves” tells me that these examples exist, but I’d much rather read more about them specifically instead of just taking the author’s word on it.

Well Cassie it looks like Ellen already took the reins and provided a great supplement to the piece! I just wanted to highlight something I linked to at the end of the article, which Ellen also linked to above. The IPCC Special Report on Global Warming of 1.5° C was released towards the end of 2018, and the Summary for Policymakers had some valuable points that I think answer your questions.

- [Subsection A.1.3] “Trends in intensity and frequency of some climate and weather extremes have been detected over time spans during which about 0.5°C of global warming occurred (medium confidence). This assessment is based on several lines of evidence, including attribution studies for changes in extremes since 1950.”

- [B.1.1] " Several regional changes in climate are assessed to occur with global warming up to 1.5°C compared to pre-industrial levels, including warming of extreme temperatures in many regions (high confidence), increases in frequency, intensity, and/or amount of heavy precipitation in several regions (high confidence), and an increase
in intensity or frequency of droughts in some regions (medium confidence). "

- [B.1.2] “Temperature extremes on land are projected to warm more than GMST (high confidence): extreme hot days in mid-latitudes warm by up to about 3°C at global warming of 1.5°C and about 4°C at 2°C, and extreme cold nights in high latitudes warm by up to about 4.5°C at 1.5°C and about 6°C at 2°C (high confidence).”

I hope this helps this helps make the case more concrete, rather than me just claiming it in the article. Happy to debate or discuss these points!

This article provides a great statistical explanation of how and why current climate change is causing extreme climatic events to increase in frequency, intensity, and extent. However, one point that I would like to stress is that while the above text states that there are not many examples of recording breaking and/or extreme weather events occurring yet, recent peer-reviewed studies and international scientific reports have illustrated that we are already in the midst of increasing extremes (and this is predicted to continue to intensify). For instance, the European Academies’ Science Advisory Council (EASAC), comprised of 27 national science academies in the EU, Norway, and Switzerland, published reports in 2013 and 2018 demonstrating that in Europe since 1980 hydrological events (such as floods) have quadrupled, and climatological (i.e. extreme droughts, forest fires, and heatwaves) and meteorological (storms) events have doubled. In 2017, Dr. Carl-Friedrich Schleussner and others published findings that extreme hot temperatures have increased globally in observational datasets (when comparing 1960-1979 values to 1991-2010). Other examples include studies on precipitation variability and intensity in North America, increases in concurrent drought and heatwaves in the Unites States, etc.